Hitherto, an injection molding machine has been known which is provided with a plurality of timing belts extended between at least one or more driven pulleys provided on one driven shaft and a driving pulley provided on each of rotation shafts of a plurality of rotation driving portions, and Patent Document 1 discloses an injection molding machine which linearly drives a screw in a heating cylinder by a plurality of injection servo motors provided with a single rotary-linear motion conversion mechanism which is connected to a rear end of the screw through a connecting member and converts a rotary motion to a linear motion and transmits the result to the screw, a single driven pulley connected to a rotary member of this rotary-linear motion conversion mechanism so as to be integrally rotated, and a plurality of timing belts which transmit rotation of a driving pulley fastened to an output shaft of each of the plurality of injection servo motors to the driven pulley.
In this type of injection molding machine, since a plurality of servo motors are used, synchronous control for each of the servo motors is required, and also, tension of each of timing belts provided in the plurality of rotary-linear conversion mechanisms needs to be set to the same. If this balance is lost, the timing belt might be fractured, the rotary shaft might be broken or the like, and malfunction, unnecessary interruption of production or the like is forced.
Therefore, a detecting device that can detect fracture of the timing belt so as to prevent occurrence of malfunction and the like is known, and Patent Document 2 discloses a belt fracture detecting device including at least one or more projections provided in the circumferential direction on the side face of the driven pulley, a proximity switch arranged so as to oppose the projections and transmits an ON-OFF signal by the projections with rotation of the driven pulley, and a controller that controls operation/stop of the motor in accordance with presence of the ON-OFF signal from this proximity switch. Patent Document 3 discloses a fail-safe device that uses a belt detection portion which detects presence of a transmission belt as an abnormality detection portion that detects abnormality of the transmission belt and displaces an engaged body by controlling a driving control portion so as to be engaged with an engaging portion.